TL neuro

March 27, 2012

Cognitive impairment in Khat users

Filed under: Cathinones, Cognition — mtaffe @ 1:52 pm

ResearchBlogging.orgA recently published study examines the cognitive effects of khat (Catha edulis) chewing. Colzato and colleagues (2011) recruited 20 khat users with 10.5 (6.5 SD) years of use, averaging 3.1 (1.8 SD) times per week. The average time spent chewing khat in each session was 5.8 hrs (1.7 SD) and all khat users met at least four criteria which define addiction under DSM-IV or ICD-10 criteria. Marijuana consumption in these individuals and the khat-free controls was about 2 joints per week, they consumed 6-8 drinks per week and had zero lifetime exposure to cocaine, amphetamines or ketamine. Subjects were also matched on IQ (using the Ravens Standard Matrices), age (~31 years), ethnicity/origin (all African) and sex (2 female in each group), although two male subjects from the khat-user group had to be excluded for excessive error rates in the switching task.

The primary behavioral measures were an N-back task (Wikipedia) and a task-switching (Wikipedia) task. The N-back is a test of working memory in which single letters are presented sequentially on a computer screen. The subject is to respond when the current letter matches the one immediately previously presented (1-back trials) or the one presented two trials before (2-back trials). The task switching task first involved blocks of trials of the discrimination of large stimulus shapes (square, rectangle) made up of lines outlined by smaller shapes with subjects to respond initially to only the “global” or “local” shape element. (Examples of similar global/local stimuli can be found here and here.) The critical test trial consisted of trials in which the response to “global” or “local” was cued across 4 trial mini-blocks with the primary outcome measures being the response time in the “switch” versus “repetition” trial types.

The study found significant reductions in accuracy within the khat user group on the 1-back (70 vs 91% correct) and 2-back (62 vs 81% correct) trials in the working memory task. Significant increases in switching cost (difference between trials where the global/local cue was the same versus when they had to switch from global to local or vice versa) were found in the khat user group compared with controls (87 vs 37 ms) in the Task Switching test as well. Together these data suggest (within the usual limits of the non-random group assignment) that chronic exposure to khat produces cognitive deficits.

The authors point to other findings of impaired inhibitory control and/or working memory in amphetamine users as evidence of a parallel effect of the active psychoactive constituents of khat which include cathinone, phenylpropanolamine and cathine (norpseudoephedrine). If so, this suggests that imaging and post-mortem investigations should be directed at dopaminergic systems of khat users.

One final caveat to the investigation is that the subjects were requested to remain drug free (including alcohol) for 24 hrs prior to the study. Given that the khat users met 4 criteria traditionally associated with addiction and dependence, it may be the case that these findings reflect acute withdrawal and cannot necessarily be inferred to represent permanent, lasting effects of chronic khat chewing.

Colzato LS, Ruiz MJ, van den Wildenberg WP, & Hommel B (2011). Khat use is associated with impaired working memory and cognitive flexibility. PloS one, 6 (6) PMID: 21698275



  1. I believe chewing Khat causes some decrease in cognitive memory and thinking. However, I wonder how much the effects of chewing Khat played into the experiment’s results. The subjects, who may or may not be addicted to a drug(s), were asked to not take any 24 hours prior. Withdrawal symptoms could play a huge factor in the results. This is a great idea for an experiment, but I think the subjects tested should not have the possibility of being addicted to other drugs. The subject should only be chewing Khat and not doing any other drugs.

    Comment by Diane Q — March 30, 2012 @ 12:21 pm

    • The subjects were pretty much only using khat, as far as stimulants go, and very little alcohol and cannabis, as far as studies which show the cognitive effects of those drugs go. Importantly, the two groups were well matched so unless one posits an interaction of khat with that (minimal) amount of alcohol or cannabis then this factor us controlled in the study.

      Comment by mtaffe — April 3, 2012 @ 5:09 pm

  2. I found it interesting that the study found that khat users had a significant reduction in accuracy of their working memory tasks. The study showed that chronic exposure to khat produces cognitive defects. This could be argued though. In the study, only African Americans were being tested and only two in the groups were female. The subjects being tested had to meet certain criteria, therefore it did not test a wide range of subjects. Also subjects had to remain drug free 24 hours prior to the study, this could have skewed their data on the subjects because they could have already started to go into withdrawal, which could skew the cognitive memory results.

    Comment by Lisa Queensen — March 31, 2012 @ 12:29 pm

    • The study was in the Netherlands, thus AfroDutch would be more appropriate, LQ. The main point here is that the groups were relatively well matched, thus isolating the effects of khat consumption. As I mentioned in the post, yes, it is possible the main effect being studied is discontinuation from khat, not the lasting effects that would be invariant no matter the length of abstinence. Obviously additional types of studies would have to be designed.

      The point here is that the results are not “skewed”. The results are constrained by the design of the study and interpretation and generalization from these results has to be done with that understanding. “Skew” implies there is some factor creating a result where one does not exist and that is not the right way to think about individual studies like this.

      Comment by mtaffe — April 3, 2012 @ 5:06 pm

  3. I learned a lot and really enjoyed reading your blog post. Especially the test about working memory in which single letters are presented sequentially on a computer screen.

    Comment by Kristina Raposa — March 31, 2012 @ 1:54 pm

  4. This article really caught my attention because a lot of people don’t think Khat has any effect on them because it’s not really a “drug” when in reality is it a drug and it does have an effect on you! It has the same effects as an amphetamine/stimulant and if those are considered abused drugs what makes this one different? Although it isnt as harmful as alcohol, or tobacco it still does some damage and can have long term effects, like depression hallucinations or oral cancer. Also, after reading this article it is also damaging to your brain! Just don’t do drugs. Stay away.

    Comment by Reama Darwish — March 31, 2012 @ 9:53 pm

    • what makes this one different?

      Well, there are at least two issues. First, just because a drug is a “stimulant” or shares certain structural elements with another amphetamine-type drug, this doesn’t mean the effects are going to be the same. The methamphetamine/MDMA distinction is one of the better known ones. MDMA has less of a compulsive use risk and the lasting toxicity to serotonin function differs from the dopaminergic toxicity issues which are more commonly reported (in animal models) for methamphetamine. Second, the route of administration and the bioavailability of the drug from the natural product can change the effects. Here, the example of coca leaves (buccal administration and poor bioavailability) versus cocaine taken intranasally is apt.

      Comment by mtaffe — April 3, 2012 @ 5:02 pm

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